Stable heat treatable nickel superalloy single crystal articles and compositions

ABSTRACT

Improved compositions for fabricating nickel superalloy single crystal articles are described. The compositions are characterized by the substantial absence of carbon, boron, zirconium and vanadium and intentional additions of cobalt. The cobalt additions increase the stability of the compositions and provide enhanced heat treatability. Single crystal articles of these compositions have utility as gas turbine engine components.

This application is a continuation of application Ser. No. 07/147,463filed Jan. 25, 1988, now abandoned, which is a divisional of applicationSer. No. 06/788,893, filed Aug. 8,1985, now abandoned, which is adivisional of application Ser. No. 06/453,202, filed Dec. 27, 1982, nowabandoned.

TECHNICAL FIELD

This invention relates to compositions which have utility as singlecrystal gas turbine engine components.

BACKGROUND ART

Single crystal gas turbine engine components offer the promise ofimproved performance in gas turbine engines. U.S. Pat. No. 3,494,709which is assigned to the assignee of the present invention, disclosesthe use of single crystal components in gas turbine engines. This patentdiscusses the desirability of limiting certain elements such as boronand zirconium to low levels. The limitation of carbon to low levels insingle crystal superalloy articles is discussed in U.S. Pat. No.3,567,526. U.S. Pat. No. 4,116,723 describes heat treated superalloysingle crystal articles which are free from intentional additions ofcobalt, boron, zirconium and hafnium. According to this patent,elimination of these elements render the compositions heat treatable.

DISCLOSURE OF INVENTION

An improved composition range is described for the production of heattreatable nickel superalloy single crystal articles. The broadcomposition contains 5%-12% chromium, 2%-8% aluminum, 0%-6% titanium,0%-9.5% tantalum, 0%-12% tungsten, 0%-3% molybdenum; 0%-3% columbium;0%-3.5% hafnium; 0%-7% rhenium; and balance essentially nickel. Thepreferred composition contains 7%-12% chromium; 3%-7% aluminum; 1%-5%titanium; 1%-8% tantalum; 0%-12% tungsten; 0%-0.8% molybdenum; 0%-3%columbium; 0%-2.5% hafnium; 0%-7% rhenium; and balance essentiallynickel. The composition is free from intentional additions of carbon,boron, zirconium and vanadium. The composition contains an intentionaladdition of cobalt sufficient to render it stable and immune to theformation of deleterious phases. The composition with the addition ofcobalt also has enhanced heat treatability; the temperature rangebetween the gamma prime solves temperature and incipient meltingtemperature is increased over that which would be possessed by acobalt-free alloy.

The foregoing, and other features and advantages of the presentinvention, will become more apparent from the following description andaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows the effect of cobalt on the micro-structural stability ofrhenium-free nickel base super-alloy compositions.

FIG. 2 shows the effect of cobalt on the micro-structural stability ofrhenium containing nickel base superalloy compositions.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is concerned with nickel base single crystalarticles which find application in aircraft gas turbine engines. Morespecifically, the invention relates to nickel base single crystalarticles containing from about 5% to about 12% chromium, from about 2%to about 8% aluminum, up to about 6% titanium, with the sum of thealuminum and titanium exceeding about 4%, up to about 9.5% tantalum, upto about 12% tungsten, up to about 3% molybdenum, up to about 3%columbium, up to about 3.5% hafnium, up to about 7% rhenium, with thesum of the molybdenum, columbium, hafnium, rhenium, tantalum andtungsten contents exceeding 5%, with the composition being free fromintentional additions of carbon, boron, zirconium and vanadium, and withthe composition containing an intentional addition of cobalt sufficientto render it stable (unless otherwise indicated, all percentage valuesare in weight percents). Preferably, the composition contains 5%-12%chromium, 3%-7% aluminum, 1%-5% titanium, 1%-8% tantalum, 0%-12%tungsten, 0%-0.8% molybdenum, 0%-3% columbium, 0%-2.5% hafnium, 0%-7%rhenium, balance essentially nickel. Preferably, the sum of the aluminumand titanium contents exceeds about 5% and the sum of the molybdenum,columbium, hafnium, rhenium, tantalum and tungsten exceeds about 10%.Further, the ratio of the titanium to aluminum is preferably less thanabout 1:1.

Chromium and aluminum contents in the amounts presented above ensurethat the alloy forms a protective alumina layer upon exposure toelevated temperatures. This type of oxidation behavior is necessary forlong component life. With less than about 5% chromium, the requiredaluminum layer will not form reliably while chromium contents in excessof about 12% tend to reduce the overall strength of the alloy. Thealuminum and titanium act together to form the gamma prime strengtheningphase (Ni₃ (Al, Ti)). The required alloy strength will be obtained whenthe sum of the aluminum and titanium exceeds 4%, and preferably about5%. The ratio of the titanium to aluminum is controlled, preferably tobe less than about 1:1; again, this helps to ensure that the desiredalumina oxide protective layer is formed. The elements tantalum,tungsten, molybdenum, columbium, hafnium and rhenium are referred to asrefractory elements and are present in the alloy for the purpose ofstrengthening. The elements tungsten, molybdenum and rhenium partitionmainly to the gamma matrix phase while the elements tantalum andcolumbium partition to the gamma prime strengthening phase. A mixture ofrefractory elements is desirable for satisfactory alloy performance andthe sum of these elements should exceed 5% and preferably 10%. Thosealloys which contain the lesser amounts of these strengthening elementswill generally be useful in vane or other nonrotating applications whilethose compositions containing the higher amounts of these strengtheningelements will find application in blades and other similar more highlystressed engine components.

These compositions find application in single crystal components whichare cast components free from internal grain boundaries. In conventionalsuperalloys, the elements of carbon, boron, zirconium are added for theprimary purpose of strengthening the grain boundaries while in singlecrystal components which contain no such grain boundaries; substantialbenefits are obtained by the substantial exclusion of these elements.Exclusion of these elements also increases the incipient meltingtemperature, thereby making it easier to solution heat treat. Thissubject is discussed at some length in U.S. Pat. No. 4,116,723. Vanadiumhas been added to certain superalloys for the purpose of gamma primeformation and minimizes the gamma prime being present as a low meltingeutectic, but causes a substantial detriment in the hot corrosionbehavior of the alloys and consequently is excluded from the presentcomposition.

The intentional addition of cobalt in closely controlled amounts is asignificant part of the present invention. Nickel base superalloys arecompositionally complex and are used in service under extreme conditionsof temperature and stress. Certain superalloys have been observed to bemicrostructurally unstable under service conditions; the terminstability relates to the formation of extraneous phases as a result oflong term exposure to service conditions. These phases are oftenreferred to as the topologically close-packed phases or TCP phases andinclude the phases, among others, referred to as sigma and mu. Thesephases are undesirable since they are generally brittle and of lowstrength, and their formation may deplete the alloy of the refractoryelements that give it strength. Consequently, their formation in ahighly stressed part in service can lead to premature catastrophicfailure. Extensive prior art investigations have related the formationof these phases to the parameter referred to as N_(v) or the electronvacancy number. A preferred method (usedin the prior art) forcalculating the N_(v) number for a superalloy matrix is given below:

1. Convert the composition from weight percent to atomic percent;

2. After long time exposure in the TCP phase forming temperature range,the MC carbides tend to transfrom to M₂₃ C₆,

a) assume one-half of the carbon forms MC in the following preferentialorder: TaC, NbC, TiC,

b) assume the remaining carbon forms M₂₃ C₆ of the followingcomposition: Cr₂₁ (Mo, W, Re)₂ C₆ or Cr₂₃ C₆ in the absence ofmolybdenum, tungsten or rhenium;

3. Assume boron forms M3B2 of the following composition: (Mo₀.5 Ti₀.15Cr₀.25 N₀.10)₃ B₂ ;

4. Assume gamma prime to be of the following composition: Ni₃ (Al, Ti,Ta, Nb, Zr, 0.5 V, 0.03 Cr*);

5. The residual matrix will consist of the atomic percent minus thoseatoms tied up in the carbide reaction, boride reaction, and the gammaprime reaction. The total of these remaining atomic percentages givesthe atomic concentration in the matrix. Conversion of this on the 100%basis gives the atomic percent of each element remaining in the matrix.It is this percentage that is used in order to calculate the electronvacancy number; and

6. The formula for calculation of the electron vacancy number is asfollows:

    (N.sub.v ref)=0.61 Ni+1.71 Co+2.66 Fe+3.66 Mn+4.66 (Cr+Mo+W+Fe)+5.66 V+6.66 Si.

As a general rule in prior art compositions, sigma phase is anticipatedwhen the N_(v) value exceeds about 2.5. The present invention arises inpart from the discovery that the relationship of N_(v) to alloyinstability is more complex than had previously been anticipated, andthat judicious additions of cobalt substantially improves the stabilityof alloys even though the N_(v) number is not substantially affected.

FIG. 1 shows the relationship between the electron vacancy number and arefractory parameter for several rhenium-free experimentally testedalloys. FIG. 1 also shows several lines which define the stable andunstable alloy regions for alloys containing various cobalt levels. FromFIG. 1, it can be seen that for a particular value of the refractoryparameter, additions of cobalt up to about 10% substantially increasethe threshold electron vacancy number at which instability occurs. Thisobservation is contrary to the prior art which had generally treated theN_(v) number as being the sole parameter controlling alloy stability.Prior art indicated that additions of cobalt would increase theinstability of the alloy.

As previously indicated, superalloys derive a substantial portion oftheir strength from the presence of solid solution strengtheners such asthe refractory metals. However, those refractory metals includingtungsten, molybdenum and rhenium which substantially partition to thematrix, also have the effect of increasing the electron vacancy number.Through the additions of cobalt as taught by the present invention inFIG. 1, high refractory element additions may be made for strengthpurposes while cobalt additions may also be made sufficient to render anunstable alloy, stable, even though they raise the electron vacancynumber (N_(v)) which prior art suggests would cause a further decreasein stability.

The alloy compositions which are shown as points in FIG. 1 are given inTable I. Compare, for example, alloy L1 and alloy 705, alloys which havesubstantially the same refractory metal content. Alloy L1 is unstable,yet alloy 705 which contains 5% cobalt is stable. Thus, it is nowpossible through the use of judicious cobalt additions to renderpreviously unstable alloys stable and suitable for long term use undersevere conditions. From FIG. 1, it can be seen that the refractorycontent affects the N_(v) level at which the alloy becomes unstable. Forcobalt-free alloys, the equation for the line separating stable andunstable alloys is about N_(v) =2.39-((W+2Mo)×0.043). Alloys with N_(v)levels in excess of this will be unstable. A significant aspect of thisinvention is the discovery that cobalt additions change the boundarybetween stable and unstable regions.

For example, alloys containing 5% cobalt are stable for N_(v)<2.74-((W+2Mo)×0.057) to a maximum of about 2.5 and alloys containing10% cobalt are stable for N_(v) <2.82-((W+2Mo)×0.058) to a maximum ofabout 2.5. Thus, a feature of this invention is the discovery of stablesingle crystal alloy compositions in the regions where:

2.39-((W+2Mo)×0.043)<N_(v) <2.82-((W+2Mo)×0.058) for alloys with 10%cobalt; and

2.39-((W+2Mo)×0.043)<N_(v) <2.74-((W+2Mo)×0.057) for alloys with 5%cobalt.

For rhenium-containing superalloys, cobalt also plays a significant rolein determining alloy stability. As taught by the present invention,sufficient additions of cobalt may be made to an unstable alloy to torender the alloy stable. Prior art would indicate that raising the levelof cobalt in an unstable alloy, thus increasing the electron vacancynumber (N_(v)) would further decrease alloy stability. As shown in FIG.2, increases in alloy stability are acquired through judicious additionsof cobalt.

The alloy compositions which are shown as points in FIG. 2 are given inTable II. Compare, for example, alloy 250 and alloy 483, alloys whichhave substantially the same refractory element content. Alloy 250 isunstable, yet alloy 483 with 5% more cobalt than alloy 250 is stableenough, though its electron vacancy number is 0.1 higher than that ofalloy 250. Thus, it is possible to control alloy stability and thusrender unstable alloys suitable for long time service under severeconditions through judicious applications of cobalt.

From FIG. 2, it can be seen that the refractory content affects theN_(v) level at which the alloy becomes unstable. For alloys containing10% cobalt, the equation for the line separating stable and unstablealloys is (composition in weight percent) N_(v) =2.56-0.027 (W+2Mo+2Re)to a maximum of about 2.5.

Alloys with N_(V) levels in excess of this will be unstable. Aspreviously shown in FIG. 1, with rhenium-free alloys, cobalt additionsto rhenium containing alloys change the boundary between stable andunstable regions.

For example, alloys containing 5% cobalt are stable for N_(v)<2.23-0.027 (W+2Mo+2Re). Thus, a feature of this invention is thediscovery of stable rhenium containing single crystal alloys in theregion of 2.23-0.027 (W+2Mo+2Re)<N_(v) <2.56-0.027 (W+2Mo+2Re) foralloys with 10% cobalt (and for N_(v) up to about 2.5).

The surprising and unexpected effect of cobalt on alloy stability standsas a notable contribution to the art permitting the development ofstable alloys with higher strength properties than those previouslyknown. However, the effect of cobalt is also substantial in anotherarea. It is now generally appreciated that maximum superalloy propertiesare obtained when the alloys are properly heat treated. Heat treatmentof superalloys involves heating to a temperature above the gamma primesolves temperature in order to dissolve the coarse, as-cast gamma primestructure followed by rapid cooling and reheating to a lower temperaturefor controlled reprecipitation of the gamma prime phase on a fine scale.Many of the complex modern superalloys have a small temperaturedifference between the required temperature for solution heat treatmentand the incipient melting temperature. This makes heat treatmentdifficult, especially on a production scale where minor compositionalvariations between metal heats cause variations in the gamma prime andincipient melting temperatures. As will be shown below, the additions ofsmall amounts of cobalt serve to increase the heat treatment range andmakes possible the heat treatment of high strength alloys which hadheretofore not been heat treatable in the absence of cobalt and thus,makes possible the achievement of the maximum strength capabilities ofthese alloys.

Each of the pairs of alloys set forth in Table III differs significantlyonly in the addition of 5% or 10% cobalt, yet in each of these cases,the cobalt addition makes a substantial change in the solution heattreatment range. The change ranges from 10° F. to 35° F. (6° C. to 19°C.) and in two cases, makes possible the heat treatment of alloys whichcould previously not be heat treated without incipient melting. Someindication as to the significance of this improved heat treatmentcapability is shown in Table IV. It should be noted that the alloys 255and 454 are outside of the scope of the present invention by virtue oftheir high tantalum content. Nonetheless, a comparison of theirproperties is instructive. Alloy 255 differs from alloy 454 in that itlacks the cobalt content of alloy 454. The incipient melting temperatureand gamma prime solves of alloy 255 are both about 2380° F. (1304.4°C.). Heat treatment at 2380° F. (1304.4° C.) of alloy 255 results insubstantial incipient melting. The rupture life of alloy 255 at 1800°F./36 ksi (982° C./25.3 kg/mm²) after heat treatment at 2380° F.(1304.4° C.) is about 40 hours, and the time to 1% creep is about 15hours. Decreasing the heat treatment temperature of alloy 255 to 2370°F. (1299° C.) effectively eliminates incipient melting, but producesonly partial heat treatment since not all of the coarse, as-cast, gammaprime phase is dissolved into the gamma solid solution. However, theeffect of even this partial solution treatment in the absence ofincipient melting is to raise the rupture life to about 53 hours and thetime to 1% creep to about 16 hours. Alloy 454 can be fully solution heattreated at 2350° F. (1288° C.) without incipient melting and after thefull solution heat treatment, the rupture life is 90 hours and the timeto 1% creep is 30 hours. This illustrates the importance of fullsolution heat treatment and the importance of avoiding incipient meltingif maximum properties are to be achieved.

Although this invention has been shown and described with respect todetailed embodiments thereof, it will be understood by those skilled inthe art that various changes in form and detail thereof may be madewithout departing from the spirit and scope of the claimed invention.

                                      TABLE I                                     __________________________________________________________________________    Effect of Cobalt on Microstructural Stability of Single Crystal               Nickel-Base Superalloys                                                       Microstructural                                                                            Composition (Weight Percent)                                     Alloy                                                                            N.sub.V.sbsb.3B                                                                  Stability                                                                            Co Cr Ti Mo W  Ta Al                                                                              Hf                                                                              V                                          __________________________________________________________________________    319                                                                              2.03                                                                             Unstable                                                                             0  8.2                                                                              .99                                                                              1.0                                                                              10.0                                                                             6.6                                                                              5.1                                                                             .1                                                                              0                                          L-1                                                                              2.08                                                                             Unstable                                                                             0  8.9                                                                              .98                                                                              0  7.9                                                                              6.0                                                                              5.9                                                                             .1                                                                              0                                          H-7                                                                              2.07                                                                             Unstable                                                                             0  9.9                                                                              1.42                                                                             1.4                                                                              6.9                                                                              5.9                                                                              5.0                                                                             .1                                                                              0                                          4A 1.89                                                                             Unstable                                                                             0  7.9                                                                              .97                                                                              2.1                                                                              9.9                                                                              3.0                                                                              5.4                                                                             .1                                                                              0                                          255                                                                              2.35                                                                             Unstable                                                                             0  9.8                                                                              1.38                                                                             0  4.1                                                                              11.9                                                                             5.2                                                                             0 0                                          454                                                                              2.43                                                                             Stable 4.9                                                                              9.6                                                                              1.6                                                                              0  4.0                                                                              12.0                                                                             4.9                                                                             0 0                                          705                                                                              2.16                                                                             Stable 5.1                                                                              8.9                                                                              .92                                                                              1.05                                                                             6.8                                                                              6.0                                                                              5.6                                                                             .1                                                                              0                                          715                                                                              2.21                                                                             Stable 5.1                                                                              10.0                                                                             .99                                                                              0  7.9                                                                              5.0                                                                              5.6                                                                             .1                                                                              0                                          718                                                                              2.18                                                                             Stable 5.1                                                                              10.0                                                                             .38                                                                              0  8.9                                                                              6.0                                                                              5.1                                                                             .1                                                                              0                                          721                                                                              2.14                                                                             Stable 5.0                                                                              9.0                                                                              .99                                                                              0  9.9                                                                              6.1                                                                              5.1                                                                             .1                                                                              0                                          A14                                                                              2.32                                                                             Stable 5.0                                                                              11.8                                                                             0  0  5.9                                                                              10.5                                                                             4.9                                                                             0 0                                          313                                                                              2.34                                                                             Stable 10.2                                                                             7.6                                                                              .93                                                                              2.02                                                                             4.1                                                                              10.4                                                                             5.0                                                                             .1                                                                              .5                                         316                                                                              2.06                                                                             Stable 10.0                                                                             5.2                                                                              1.52                                                                             2.04                                                                             7.0                                                                              5.1                                                                              5.6                                                                             .1                                                                              .4                                         __________________________________________________________________________

                                      TABLE II                                    __________________________________________________________________________    Effect of Cobalt on                                                           Microstructural Stability of Rhenium Containing Nickel-Base Superalloys       Microstructural                                                                            Composition (Weight Percent)                                     Alloy                                                                            N.sub.V.sbsb.3B                                                                  Stability                                                                            Re Co Cr                                                                              Ti                                                                              Mo W Ta Al                                                                              Hf                                                                              V                                          __________________________________________________________________________    483                                                                              2.22                                                                             Stable 2.8                                                                              10.0                                                                             7.5                                                                             0 2.0                                                                              3.0                                                                             10.2                                                                             5.3                                                                             .1                                                                              0                                          433                                                                              2.17                                                                             Stable 3.0                                                                              10.0                                                                             4.9                                                                             0 2.3                                                                              3.0                                                                             11.9                                                                             5.2                                                                             0 .7                                         249                                                                              2.15                                                                             Unstable                                                                             1.9                                                                              4.8                                                                              7.6                                                                             0 1.9                                                                              2.2                                                                             12.0                                                                             5.0                                                                             0 .8                                         250                                                                              2.12                                                                             Unstable                                                                             2.9                                                                              4.7                                                                              7.6                                                                             0 1.9                                                                              1.2                                                                             11.9                                                                             5.0                                                                             0 .7                                         305                                                                              2.05                                                                             Unstable                                                                             3.0                                                                              4.9                                                                              4.8                                                                             .9                                                                              2.1                                                                              4.0                                                                             12.2                                                                             5.0                                                                             0 0                                          421                                                                              1.85                                                                             Stable 3.0                                                                              5.1                                                                              5.0                                                                             0 2.1                                                                              1.1                                                                             11.7                                                                             5.2                                                                             0 .7                                         422                                                                              2.06                                                                             Stable 3.0                                                                              10.0                                                                             4.9                                                                             0 2.1                                                                              1.0                                                                             11.7                                                                             5.4                                                                             0 .7                                         423                                                                              2.22                                                                             Stable 2.9                                                                              15.0                                                                             4.9                                                                             0 2.1                                                                              1.0                                                                             12.0                                                                             5.3                                                                             0 .7                                         __________________________________________________________________________

                                      TABLE III                                   __________________________________________________________________________    Effect of Cobalt on Heat Treatment Response of Single Crystal Nickel-Base     Superalloys                                                                      Gamma                                                                             Incipient                                                                           Solution Heat                                                       Prime                                                                             Melting                                                                             Treatment                                                           Solvus                                                                            Temperature                                                                         Range  Composition (Weight Percent)                              Alloy                                                                            (° F.)                                                                     (° F.)                                                                       (° F.)                                                                        Cr                                                                              Ti Mo W Ta Al                                                                              Co                                                                              Re                                                                              Hf                                     __________________________________________________________________________    255                                                                              2400                                                                              2380  -20    9.8                                                                             1.38                                                                             0  4.1                                                                             11.9                                                                             5.2                                                                             0 0 0                                      454                                                                              2350                                                                              2365  15     10                                                                              1.5                                                                              0  4 12 5 5 0 0                                      706                                                                              2355                                                                              2410  55     9.0                                                                             .96                                                                              1.05                                                                             7.0                                                                             5.8                                                                              5.5                                                                             0 0 .1                                     705                                                                              2330                                                                              2405  75     8.9                                                                             .92                                                                              1.05                                                                             6.8                                                                             6.0                                                                              5.6                                                                             5.1                                                                             0 .1                                     301                                                                              2385                                                                              2390   5     7.5                                                                             .96                                                                              1.64                                                                             3.7                                                                             11.8                                                                             5.0                                                                             4.9                                                                             0 0                                      302                                                                              2355                                                                              2375  20     7.6                                                                             1.01                                                                             1.87                                                                             3.8                                                                             11.8                                                                             5.1                                                                             9.9                                                                             0 0                                      305                                                                              2425                                                                              2415  -10    4.8                                                                             .94                                                                              2.1                                                                              4.0                                                                             12.2                                                                             5.0                                                                             4.9                                                                             3.0                                                                             0                                      306                                                                              2395                                                                              2395   0     4.9                                                                             .94                                                                              2.1                                                                              4.2                                                                             11.8                                                                             5.0                                                                             9.9                                                                             3.0                                                                             0                                      __________________________________________________________________________

                  TABLE IV                                                        ______________________________________                                        1800° F./36 ksi                                                        Alloy     Heat Treatment   Life     1%                                        ______________________________________                                        255       2380° F.                                                                         (Melting)  40.5 hrs 18.1 hrs                              (10 Cr, 1.5 Ti,                44.0     13.4                                  5 Al, 4 W,                     38.8     12.9                                  12 Ta, Bal. Ni)                                                                         2370° F.                                                                         (Partial   51.4     16.0                                                      Solutioning)                                                                             55.0     17.0                                  454       2350° F.                                                                         (Fully     90       30                                    (10 Cr, 5 Co,       Solutioned)                                               1.5 Ti, 5 Al,                                                                 4 W, 12 ta,                                                                   Bal. Ni)                                                                      ______________________________________                                    

We claim:
 1. A composition useful in the production of single crystalturbine articles consisting of:a. from about 5% to about 12% chromium;b. from about 2% to about 8% aluminum; c. up to about 6% titanium, withthe sum of the aluminum and titanium contents being at least 4% and theratio of aluminum to titanium being at least 1:1; d. up to about 9.5%tantalum; e. up to about 12% tungsten; f. up to about 3% molybdenum; g.up to about 3% columbium; h. up to about 3.5% hafnium; i. up to about 7%rhenium; j. with the sum of molybdenum, columbium, hafnium, rhenium,tantalum and tungsten exceeding 5%; k. said composition being free fromintentional additions of carbon, boron, zirconium and vanadium; l. anintentional addition of cobalt sufficient to render the compositionstable according to the criteria set out in FIGS. 1 or 2, wherein theaddition of cobalt is from 1-10%; balance essentially nickel.
 2. Acomposition as in claim 1 containing 7%-12% chromium, 3%-7% aluminum,1%-5% titanium, 1%-8% tantalum, and up to 2.5% hafnium.
 3. A compositionas in claim 1 in which the sum of the Ta+W+Mo+Cb+Hf+Re exceeds about10%.
 4. A single crystal article having a good combination of propertiesat elevated temperature which comprises:a. from about 7% to about 12%chromium; b. from about 2% to about 8% aluminum; c. up to about 6%titanium, with the sum of the aluminum and titanium contents being atleast 4% and the ratio of aluminum to titanium being at least 1:1; d.from about 1.0% to about 9.5% tantalum; e. from about 2% to about 12%tungsten; f. up to about 3% molybdenum; g. up to about 3% columbium; h.up to about 3.5% hafnium; i. up to about 7% rhenium; j. with the sum ofmolybdenum, columbium, hafnium, rhenium, tantalum and tungsten exceeding5%; k. said composition being free from intentional additions of carbon,boron, zirconium and vanadium; l. an intentional addition of cobaltsufficient to render the composition stable according to the criteriaset out in FIGS. 1 or 2, wherein the addition of cobalt is from 1-10%;balance essentially nickel, said article being free from internal grainboundaries, said article being heat treatable as a result of having anincipient melting temperature which is higher than the gamma primesolves temperature.
 5. A heat treated single crystal gas turbinecomponent having good properties at elevated temperature whichcomprises:a. from about 7% to about 12% chromium; b. from about 2% toabout 8% aluminum; c. up to about 6% titanium, with the sum of thealuminum and titanium contents being at least 4% and the ratio ofaluminum to titanium being at least 1:1; d. from about 1.0% to about9.5% tantalum; e. from about 2% to about 12% tungsten; f. up to about 3%molybdenum; g. up to about 3% columbium; h. up to about 3.5% hafnium; i.up to about 7% rhenium; j. with the sum of molybdenum, columbium,hafnium, rhenium, tantalum and tungsten exceeding 5%; k. saidcomposition being free from intentional additions of carbon, boron,zirconium and vanadium; l. an intentional addition of cobalt sufficientto render the composition stable according to the criteria set out inFIGS. 1 or 2, wherein the addition of cobalt is from 1-19%; balanceessentially nickel, said component having a uniform fine distribution ofthe gamma prime phase, with an average gamma prime particle dimension ofless than about 0.4 micron.
 6. A method of rendering an otherwise phaseunstable rhenium-free nickel base superalloy phase stable whichcomprises adding from 1-10% cobalt in accordance with FIG.
 1. 7. Amethod of rendering an otherwise phase unstable rhenium containingnickel base superalloy phase stable which comprises adding from 1-10%cobalt in accordance with FIG.
 2. 8. A method of rendering an otherwisephase unstable rhenium-free nickel base superalloy phase stable, saidalloy consisting essentially of:a. from about 7% to about 12% chromium;b. from about 2% to about 8% aluminum; c. up to about 6% titanium, withthe sum of the aluminum and titanium contents being at least 4% and theratio of aluminum to titanium being at least 1:1; d. from about 1.0% toabout 9.5% tantalum; e. from about 2% to about 12% tungsten; f. up toabout 0.8% molybdenum; g. up to about 3% columbium; h. up to about 3.5%hafnium; i. with the sum of molybdenum, columbium, hafnium, tantalum andtungsten exceeding 5%; j. said composition being free from intentionaladditions of carbon, boron, zirconium and vanadium; balance essentiallynickel, which comprises: adding from 1-10% cobalt in accordance withFIG. 1 whereby the alloy will be rendered phase stable.
 9. A method ofrendering an otherwise phase unstable rhenium containing nickel basesuperalloy phase stable, said alloy consisting essentially of:a. fromabout 7% to about 12% chromium; b. from about 2% to about 8% aluminum;c. up to about 6% titanium, with the sum of the aluminum and titaniumcontents being at least 4% and the ratio of aluminum to titanium beingat least 1:1; d. from about 1.0% to about 9.5% tantalum; e. from about2% to about 12% tungsten; f. up to about 0.8% molybdenum; g. up to about3% columbium; h. up to about 3.5% hafnium; i. an intentional addition ofrhenium of up to 7%; j. with the sum of molybdenum, columbium, hafnium,rhenium, tantalum and tungsten exceeding 5%; k. said composition beingfree from intentional additions of carbon, boron, zirconium andvanadium; balance essentially nickel, which comprises: adding from 1-10%cobalt in accordance with FIG. 2 whereby the alloy will be renderedphase stable.
 10. A method of rendering an otherwise phase unstablerhenium-free nickel base superalloy phase stable, said alloy consistingessentially of:a. from about 7% to about 12% chromium; b. from about 2%to about 8% aluminum; c. up to about 6% titanium, with the sum of thealuminum and titanium contents being at least 4% and the ratio ofaluminum to titanium being at least 1:1; d. from about 1.0% to about9.5% tantalum; e. from about 2% to about 12% tungsten; f. up to about 3%molybdenum; g. up to about 3% columbium; h. up to about 3.5% hafnium; i.with the sum of molybdenum, columbium, hafnium, tantalum and tungstenexceeding 5%; j. said composition being free from intentional additionsof carbon, boron, zirconium and vanadium; balance essentially nickel;which comprises: adding cobalt in accordance with FIG. 1 wherein saidalloy is stable for N_(v) <2.82-((W+2Mo)×0.058) for a 10% addition ofcobalt and said alloy is stable for N_(v) <2.74-((W+2Mo)×0.057) for a 5%addition of cobalt.
 11. A method of rendering an otherwise phaseunstable rhenium containing nickel base superalloy phase stable, saidalloy consisting essentially of:a. from about 7% to about 12% chromium;b. from about 2% to about 8% aluminum; c. up to about 6% titanium, withthe sum of the aluminum and titanium contents being at least 4% and theratio of aluminum to titanium being at least 1:1; d. from about 1.0% toabout 9.5% tantalum; e. from about 2% to about 12% tungsten; f. up toabout 3% molybdenum; g. up to about 3% columbium; h. up to about 3.5%hafnium; i. an intentional addition of rhenium of up to 7%; j. with thesum of molybdenum, columbium, hafnium, rhenium, tantalum and tungstenexceeding 5%; k. said composition being free from intentional additionsof carbon, boron, zirconium and vanadium; balance essentially nickel;which comprises: adding cobalt in accordance with FIG. 2 wherein saidalloy is stable for N_(v) <2.56-0.027 (W+2Mo+2Re) for a 10% addition ofcobalt and said alloy is stable for N_(v) <2.23-0.027 (W+2Mo+2Re) for a5% addition of cobalt.
 12. A composition useful in the production ofsingle crystal turbine articles comprised of:a. about 5% chromium; b.from about 2% to about 8% aluminum; c. up to about 6% titanium, with thesum of the aluminum and titanium contents being at least 4% and theratio of aluminum to titanium being at least 1:1; d. up to about 9.5%tantalum; e. up to about 1% tungsten; f. up to about 2.1% molybdenum; g.up to about 3% columbium; h. up to about 3.5% hafnium; i. up to about 3%rhenium; j. with the sum of molybdenum, columbium, hafnium, rhenium,tantalum and tungsten exceeding 5%; k. said composition being free fromintentional additions of carbon, boron, zirconium and vanadium; l.between about 10% and about 15% cobalt; balance essentially nickel. 13.A composition as in claim 12 in which the sum of the Ta+W+Mo+Cb+Hf+Reexceeds about 10%.